1. Field of the Invention
The present invention generally relates to a roll coating apparatus for applying a coating on a surface of a substrate by means of a roller. More specifically, the present invention relates to a roll coating apparatus for applying a coating on an uneven surface of a thin plate substrate.
2. Description of the Prior Art
Roll coaters have been well known. A roll coater is used for applying a photoresist liquid to form a polysilicon pattern on a surface of a substrate such as a semiconductor and the like. In manufacturing a thermal head, for example, a metal film is deposited on a glass substrate, and a photoresist pattern is formed on the metal film utilizing a roll coater. FIG. 1 shows a schematic cross section of the roll coater. Referring to FIG. 1, a conventional roll coater comprises a coating roll 1 for applying a resist liquid R on a substrate 5, a doctor roll 2 in contact with the coating roll 1 to apply the resist liquid R to the coating roll, and a back up roll 3 adjacent to the coating roll 1 for conveying the substrate 5. The resist liquid R is supplied through a resist liquid supplying apparatus 6 to a space defined by the coating roll 1 and the doctor roll 2. The amount of the resist liquid R supplied on the coating roll 1 is adjusted by adjusting the pressure with which the doctor roll 2 pressing against the coating roll 1. As a result a constant thickness of resist liquid R is applied to the surface of the substrate 5 inserted between the coating roll 1 and the back up roll 3.
FIG. 2 shows a cross section of the substrate 5. Referring to FIG. 2, an aluminum metal thin film having the thickness of about 1 .mu.m is deposited on the surface of the substrate 5. Prescribed portions of the metal thin film are etched to form metal thin film pieces 9 on the substrate 5. The resist liquid R is applied over the surface of the substrate 5 and the metal thin film pieces 9 to a thickness of about 4 .mu.m by means of the roll coater. Consequently, air cavities 10 may be formed around the outer periphery of the metal thin film pieces 9. If the resist liquid R has high viscosity, a large number of cavities 10 are generated. In that cast, the film formed of the resist liquid R tears in the succeeding step of drying, thereby forming pin holes. Consequently, the quality of the substrate is degraded.
In order to solve the above described problem, the applicant of the present application discloses a method in Japanese Patent Laying-Open Gazette Number 209676/1984. FIG. 3 shows an arrangement of the roll coaters disclosed in the foregoing publication. Referring to FIG. 3, roll coaters A1 and A2 are arranged successively along a conveyer path of the substrate 5. First grooves are formed on the surface of a coating roll 1 of the inlet side roll coater A1. Second grooves are formed on the surface of a coating roll 1' on the outlet side roll coater A2. FIG. 4A shows the coating roll of the inlet side roll coater A1 and FIG. 4B is an enlarged view of the portion shown by IVB of FIG. 4A. Referring to FIGS. 4A and 4B, the first grooves each have a width of W.sub.1. FIG. 5A shows the outlet side roll coater A2 and FIG. 5B is an enlarged view of the portion shown by VB of FIG. 5A. Referring to FIGS. 5A and 5B, each of the second grooves have a width of W.sub.2. Referring to FIGS. 4B and 5B, the width W.sub.2 of the groove 11' of the outlet side roll coater is greater than the width W.sub.1 of the groove 11 of the inlet side roll coater A1. Since the width W.sub.2 of the grooves 11' of the coating roll of the outlet side roll coater A2 is greater than the width W.sub.1 of the inlet side roll coater A1, more coating liquid is applied to the substrate 5 by coater A2 than coater A1. Specifically, if the depth of the groove is constant, the amount of the coating liquid applied to the substrate 5 is in proportion to the width of the groove. In accordance with the roll coating method shown in FIG. 3, a thin film of the coating liquid R is formed by the inlet side roll coater A1. Therefore, even if a cavity 10 is generated upon application of the thin layer of coating liquid R by the inlet side roll coater A1, the air in the cavity easily escapes. Thereafter, a thicker film of the coating liquid is applied on the substrate 5 by the coating roll 1' of the outlet side roll coater A2. Since there is already a cavityless thin layer of coating liquid R on the substrate 10 the surface tension of the coating liquid causes the thicker layer of coating liquid R applied by the outlet side roll coater A2 to be evenly applied without the formation of an cavities. Consequently, a coating having a desired uniform film thickness is formed on the substrate.
Meanwhile, an article entitled .cent.Manufacturing Process of a Liquid Crystal Color Panel" is reported in SEMICONDUCTOR WORLD, July, 1987, pp. 160-165. FIGS. 6 and 7 are schematic cross sectional views illustrating the manufacturing process of a polysilicon TFT (Thin Film Transistor) for liquid crystal disclosed in the article.
Referring to FIGS. 6 and 7, the manufacturing process of the TFT disclosed in the above mentioned article will be schematically described. On a main surface of a transparent glass substrate L.sub.1, a polysilicon film L.sub.2 having source and drain regions, a gate insulating film (SiO.sub.2) L.sub.3, a second polysilicon film L.sub.4 which will serve as a gate electrode, and an interlayer insulating film (SiO.sub.2) L.sub.5 are formed as shown by solid lines in FIG. 6. An indium tin oxide film (ITO film) L.sub.6 is formed as shown by a two dotted lines of FIG. 6, in order to form a connection (a portion shown by L.sub.61 in FIG. 7) for driving signals and a pixel electrode for a liquid crystal display (a portion shown by L.sub.62 in FIG. 7). A photoresist L.sub.7 is applied on the surface of the indium tin oxide film L.sub.6. Consequently, a connection portion L.sub.61 for driving signals and a pixel electrode L.sub.62 for the LCD are formed on the polysilicon film L.sub.2 having the source and drain regions, as shown in FIG. 7.
The succeeding steps of manufacturing the TFT have no relation with the present invention so that the description thereof will be omitted.
As shown in FIG. 6, a contact hole H is formed in the connection portion. The contact hole H has a diameter of several .mu.m to several 10 .mu.m and a depth of several .mu.m. The coating liquid (such as a resist) can not be applied on the above mentioned contact hole H by any of the conventional roll coaters shown in FIGS. 1, 3, 4A, 4B, 5A and 5B. Consequently, pin holes are generated.
FIG. 8 illustrates the main causes of the above described pin holes. As described with reference to FIGS. 4B and 5B, the conventional coating roll has grooves. Sometimes the groove of the coating roll coincides with the position of the contact hole and sometimes it does not. FIG. 8 shows a case in which a concave air hole 19 formed in the coating liquid R in the groove 11 of the coating roll 1 coincides with the position of the contact hole H. In this case, the contact hole H is not filled with the coating liquid R. Consequently, a pin hole may possibly be generated.
If a pin hole is generated, there will be a defect in at least one of the signal line L.sub.61 and the pixel electrode L.sub.62 shown in FIG. 7, and the circuit will not operate.